The present invention relates to a progressive-power lens (progressive multifocal lens) for use in spectacles for both near and far use, and more particularly to a progressive-power lens affording comfortable wear even when the curve of the lens is shallow, and to a design process therefor.
Spectacles with progressive-power lenses that function as spectacles for both near and far use are used in cases where the ability of the eyes to adjust has diminished due to presbyopia, for example. A progressive-power lens comprises an upper far zone for distance vision and a lower near zone for near vision. The intermediate portion between the far zone and near zone is for intermediate vision.
Typically, of the two faces, namely front and back faces of a progressive-power lens, the front face, directed towards objects, is the progressive face on which the refractive power (hereinbelow, refractive power is occasionally referred to as xe2x80x9cpowerxe2x80x9d) changes progressively, and the back face, directed towards the eyes, is of relatively simple prescription, for example, spherical or astigmatic. The surface refractive power of the distance portion on the progressive face is termed the xe2x80x9cbase curvexe2x80x9d and is used as a reference face when designing the progressive face. Accordingly, progressive faces with substantially identical progressive characteristics can be obtained with different base curves, at least insofar as surface refractive power is concerned. Typically, a deeper base curve (i.e., a smaller radius of curvature on the curving face) is associated with greater center thickness in a (+) lens, or with greater edge thickness in a (xe2x88x92) lens. Conversely, a shallower base curve allows the lens to be made thinner, but presents certain disadvantages in terms of power (transmission power) and aberration (transmission aberration).
When the eye travels from a visual target at infinite distance ahead to a visual target closer ahead, wearing these spectacles, a continuous line of the set of points of intersection of the line of vision with the lens can be assumed. This line is typically termed the xe2x80x9cprincipal sight line,xe2x80x9d xe2x80x9cprincipal meridonal line,xe2x80x9d or xe2x80x9ccenter reference line,xe2x80x9d It is a line that extends vertically through the center of the lens so as to divide it into substantially left and right portions; of the front and back faces of the lens, the line is defined on the progressive face where progressive action is present.
The assessment of a progressive-power lens is typically performed in terms of analysis of the level of error in refractive power and/or the level of astigmatism along the principal sight line, as well as the refractive power error distribution and astigmatism distribution in the zones to the left and right of the principal sight line, etc. The criteria for conventional evaluation generally dictate that astigmatism on the principal sight line be zero. That is, there is a series of minute spherical faces on the principal sight line; such a principal sight line is sometimes termed a xe2x80x9cumbilical curvature.xe2x80x9d
Even if it is possible to eliminate the refractive power error or astigmatism of a spectacle lens per se in an ideal manner, this will not necessarily make it possible to eliminate aberrations such as astigmatism, curvature of field, etc., which are caused in an image produced in the eye when the spectacle lens is worn. It is assumed that the quality of sight which is obtained by wearing spectacles is determined, after all, by such aberrations of the intraocular and not solely by the optical characteristics of the lens surface. In other words, aberrations in intraocular image and the like are also affected by various factors including the position on the lens through which the line of sight passes, the distance from the lens to the center of rotation of the eye, the refractive power of the ocular lens, and the angle of rotation of the eye. Accordingly, there will be cases when, even if the astigmatism on the lens surface is zero, the astigmatism of the intraocular image obtained when the lens is worn will not be zero. Conversely, it will also be found that, when attempts are made to obtain zero aberration of intraocular image, the astigmatism on the lens surface will assume a value other than zero.
Japanese Patent Publication S47-23943 discloses a method for improving astigmatism and aberration (e.g., curvature of field, distortion, etc.) in distance vision, intermediate vision, and near vision with spectacle wear. Where the correction affording the improvement is implemented on the progressive face, surface astigmatism along the principal sight line on the progressive face is not zero; in other words, this progressive-power lens has a so-called xe2x80x9cnon-umbilical curvaturexe2x80x9d portion.
In Japanese Patent Applications Laid-Open (JP-A) S56-78817, Japanese PCT Patent Application Laid-Open H4-500870, JP-A H6-18823, JP-A H8-136868, Patent Application S57-170627 and the four divisional applications thereof, and elsewhere are disclosed inventions purporting to improve astigmatism during spectacle wear.
These publications suggest that, when the surface curve of a spectacle lens is designed, it is necessary to take into consideration the astigmatism of intraocular image, etc. observed when spectacles are worn. However, research carried out by the inventors of the present invention has shown that the plurality of error factors such as astigmatism, etc. occurring with spectacle wear include factors which cancel each other out, and that good results cannot be obtained through the simple approach of minimizing the respective error factors.
With respect to this point, during wear of the aforementioned progressive-power lens, where the intraocular image of an object point viewed through an arbitrary location on the spectacle lens is not focused at a single point, but rather imaged as two line segments separated by an interval, let the difference of the two refractive powers for these two focal locations be defined as transmission astigmatism at the aforesaid location on the spectacle lens; and let the difference between the average value of refractive powers for the two focal locations and the refractive power for the correct focal location be defined as the transmission average refractive power error (curvature of field). Where correction is performed, for example, so as to simply minimize the astigmatism in intraocular image (i.e., transmission astigmatism) in the principal zone of the lens, or at least in the principal zone on the principal sight line, the error resulting from focusing of the image at a location diverging from the correct focal location in the eye (i.e., transmission average refractive power error) will increase, and this will often result in decreasing the wear comfort.
Accordingly, it is important, when designing a lens, to consider the balance between the astigmatism and the transmission average refractive power error. However, it has been found that an ideal balance cannot be achieved by such a simple approach as to make these two equivalent in the principal zone of the lens. Particularly where a shallow base curve is employed in order to make the lens lighter and thinner, the distance between the eye and the distance portion increases, as does the angle of the normal between the visual axis and the lens face, resulting in a marked increase in transmission astigmatism and the transmission average refractive power error. Accordingly, greater correction is required, and this correction has the drawback of creating side effects that can in turn cause aberration to increase.
The present invention was developed with the foregoing in view, and has as an object to provide a progressive-power lens affording superior wear comfort, even where a shallow base curve is employed in order to make the lens lighter and thinner, and a design method for such a lens.
To solve the aforementioned problems, the first invention is a progressive-power lens comprising a distance portion formed in the upper region of the lens; a near portion formed in the lower region of the lens; and an intermediate portion situated in an intermediate portion between said distance portion and near portion; said lens having a refractive power distribution wherein the refractive power of each portion changes progressively going from said distance portion through the intermediate portion to the near portion along a principal sight line that is a center reference line dividing the lens into substantially left and right portions; and said distance portion having a predetermined addition range corresponding to the prescription for correcting hyperopia and that for correcting myopia; wherein, where said lens is for correcting hyperopia, having positive refractive power in said distance portion, the correction provided thereby has the effect of minimizing transmission astigmatism at each point on said principal sight line; and where said lens is for correcting myopia, having negative refractive power in said distance portion, the correction provided thereby has the effect of minimizing the transmission average refractive power error at each point on said principal sight line; and a method for designing such a lens.
The second invention is a progressive-power lens according to the first invention, wherein the correction provided thereby has the effect of minimizing transmission astigmatism at each point on the principal sight line in said near portion, and method for design thereof.
The third invention is a progressive-power lens according to the first invention, wherein the correction provided thereby has the effect of minimizing and transmission average refractive power error at each point on the principal sight line in said near portion, and method for design thereof.
The fourth invention is a progressive-power lens according to the any of the first to third inventions, wherein no such correction is made that transmission astigmatism becomes zero at either the distance reference point or the near reference point, or at both reference points, and method for design thereof.
The inventions having the constitutions described hereinabove are radically different from conventional methods in that ultimate wear comfort has been designed taking into consideration not only the transmission astigmatism and the transmission average refractive power error, but also the accommodating power of the eye with respect thereto. Of the two refractive powers mentioned in the preceding definition of transmission astigmatism, one refractive power is defined as transmission refractive power (power) in the meridional cross section and shall be referred to as transmission meridional refractive power (power), etc., while the other is defined as transmission refractive power (power) in the sagittal cross section, and shall be referred to as transmission sagittal refractive power (power) or sagittal direction transmission refractive power (power), etc.
The above definition is made based on the fact that the principal sight linexe2x80x94the center reference line that divides the lens into substantially left and right portionsxe2x80x94extends substantially in the meridional direction and, therefore, in order to simplify the following description, the direction in which the principal sight line extends is referred to as xe2x80x9cmeridionalxe2x80x9d, while the direction perpendicular thereto is referred to as xe2x80x9csagittalxe2x80x9d. The two refractive powers of transmission astigmatism in zones other than that on the principal sight line are not necessarily limited to the meridional and sagittal directions.
For example, where transmission astigmatism is present, this cannot be rectified by the accommodating power of the eye (in other words, the effects of transmission astigmatism are in the nature of an absolute value). However, for a transmission average refractive power error (curvature of field), it may be corrected by the accommodating power of the eye, depending on whether the error is positive or negative: an error is positive (+) when the value thereof is larger than the inherent refractive power, and is negative (xe2x88x92) when it is smaller.
Typically, a correct lens power for distance vision is a refractive power that can focus on the retina an image of the object point at a distance of infinity, independently of the accommodating ability of the eye. Accordingly, when an object point at a distance of infinity is viewed wearing a spectacle lens of this power, the ocular lens will be in its thinnest state. As this object point gradually moves from infinity to a position closer to the eyeball, if the ocular lens does not adjust, the focus location of the object point will diverge from the retina so that the image becomes blurred. However, the accommodating ability of the eye works automatically to swell the ocular lens so that the image of the object point remains focused on the retina.
Since the ocular lens is in its thinnest state when viewing a distance independently of the accommodating ability of the eye (with the naked eye), the refractive power of the ocular lens cannot be weakened any more, but it can be strengthened. In other words, during distance vision, the accommodating power of the eye has the effect of increasing the thickness of the ocular lens, or of increasing the refractive power (hereinbelow referred to as xe2x80x9cpositive (+) effect xe2x80x9d), but does not have the effect of decreasing the thickness of the ocular lens, or of decreasing the refractive power of the ocular lens (hereinbelow occasionally referred to as xe2x80x9cnegative (xe2x88x92) effectxe2x80x9d).
Accordingly, in the case of distance vision, in the event of error such that average refractive power in transmission deviates towards refractive power that is larger than the original refractive power (hereinbelow referred to as positive (+) transmission average refractive power error), this error cannot be cancelled out by the accommodating power of the eye. However, conversely, in the event of error such that average refractive power in transmission deviates towards refractive power that is lower than the original refractive power (hereinbelow referred to as negative (xe2x88x92) transmission average refractive power error), this error can be cancelled out automatically by the accommodating power of the eye.
Here, let it be assumed that an existing progressive-power lens having a relatively deep base curve is given a shallower base curve in order to make it thinner. In many cases, simply making the base curve thinner will increase transmission astigmatism and transmission average refractive power error, etc. in each portion of the lens. Accordingly, correction is performed so as to minimize this these errors.
One such possible correction is correction performed so as to minimize transmission astigmatism in the distance portion. Where correction performed so as to minimize transmission astigmatism in the distance portion of a lens with positive distance-vision power (a hyperopia corrective lens with a distance portion having positive refractive power), in many cases the average refractive power in transmission will assume a (xe2x88x92) error. However, as noted, this error can be compensated for by the accommodating power of the eye per se provided it is within a predetermined range.
In contrast, correction performed so as to minimize transmission astigmatism in the distance portion of a lens with a negative distance-vision power (a myopia corrective lens with a distance portion having negative refractive power), in many cases the average refractive power in transmission will assume a (+) error. However, as noted, this error cannot be compensated for by the accommodating ability of the eyeball per se. Accordingly, it will be apparent that the method of correction such that transmission astigmatism in the distance portion is minimized, while affording minimization of transmission astigmatism that per se represents an improvement, also creates a new problem of (+) transmission average refractive power error (curvature of field).
To summarize the results of the preceding explication, for a hyperopia corrective lens with a distance portion having positive refractive power, the design will incorporate correction such that transmission astigmatism at each point on the principal sight line is minimized, whereas for a myopia corrective lens with a distance portion having negative refractive power, it will incorporate correction such that transmission average refractive power error at each point on the principal sight line is minimized.
Let us now consider improvements to the near portion. As regards the near portion, preferred practice is to correct so as to minimize transmission astigmatism or correct so as to minimize transmission average refractive power error, regardless of whether the lens is for correcting hyperopia or for correcting myopia. On the one hand, it will be apparent that where correction minimizes transmission astigmatism in the near portion of the aforementioned xe2x80x9cprogressive-power lens with a shallow base curve,xe2x80x9d the transmission average refractive power error (curvature of field) in the near portion will be (xe2x88x92) for one with positive distance-vision power and conversely (+) for a negative one.
In other words, this means that for one with positive distance-vision power, addition will act weakly, while for a negative one it will act strongly. Here, where addition is weak, the original function is not adequate, whereas if stronger than needed, the associated distortion will also be stronger than necessary. Accordingly, the choice exists to improve transmission average refractive power error in the distance portion and near portion while leaving transmission astigmatism in the reference points of the distance portion and near portion completely uncorrected.